Introduction to Antilock Braking


What is an ABS?

Antilock braking systems (ABSs) are electronic systems that monitor and control wheel slip during vehicle braking. ABSs can improve vehicle control during braking, and reduce stopping distances on slippery (split or low coefficient of friction) road surfaces by limiting wheel slip and minimizing lockup. Rolling wheels have much more traction than locked wheels. Reducing wheel slip improves vehicle stability and control during braking, since stability increases as wheel slip decreases. ABSs can be applied to nearly all types of vehicles and can be successfully integrated into hydraulic and air brake systems (including air over hydraulic).

An ABS consists of several key components: electronic control unit (ECU), wheel speed sensors, modulator valves, and exciter rings. Here’s how these components work together:

1. Wheel speed sensors constantly monitor and send electrical pulses to the ECU at a rate proportional to the wheel speed.

2. When the pulse rates indicate impending wheel lockup, the ECU signals the modulator valve(s) to reduce and/or hold the brake application pressure to the wheel(s) in question.

3. The ECU then adjusts pressure, seeking one which gives maximum braking without risking wheel lockup.

4. When the ECU acts to modulate the brake pressure, it will also (on most vehicles) turn off the retarder (if so equipped) until the risk of lockup is over.

5. The ECU continually checks itself for proper operation. If it detects a malfunction/failure in the electrical/electronic system, it will shut down that part of the ABS affected by the problem—or the entire ABS—depending upon the system and the problem. When this happens, the ABS malfunction lamp lights. An ABS adjusts brake pressure much faster and more accurately than can drivers. It’s faster because:

• electronic controls are very fast and

• ABS modulator valves are physically closer to the brakes than is the driver’s foot brake valve.

It is more effective, too, because an ABS can tailor the brake pressure to each wheel or set of wheels to provide maximum braking/stability.

Modern antilock braking systems all feature the following major components

• Electronic Control Unit (ECU)

• Modulator Valves

• Wheel Speed Sensors (pickup and exciter)

• ABS Malfunction Indicator Lamps • Diagnostics


Electronic Control Unit (ECU)

The ECU processes all ABS information and signal functions. It receives and interprets voltage pulses generated by the sensor pickup as the exciter teeth pass by, and uses this information to determine:

• impending wheel lock-up and

• when/how to activate the ABS modulator valves.

The ECU connects to the following ABS components: wheel speed sensors, ABS modulator valves, power source, ground, warning lamps, blink code switch, J1587* diagnostic connector, and retarder control device (usually by relay or the J1922**/ J1939*** datalink.) The ECU also makes self-diagnostic checks during normal operation.

During braking, the ECU uses voltage pulses from each wheel speed sensor to determine wheel speed changes. If the ECU determines that the pulse rate of the sensed wheels indicates imminent lock-up, it cycles the ABS modulator valves to modify brake air pressure as needed to provide the best braking possible.

The ECU sends signals to the ABS malfunction indicator lamp or blink code lamp to communicate ABS faults. It also sends signals to the retarder control to disengage the retarder when the ABS is working. When the ABS stops modulating the brake pressure, the ECU permits retarder use once again.

Modulator Valves


ABS modulator valves regulate the air pressure to the brakes during ABS action. When not receiving commands from the ECU, the modulator valve allows air to flow freely and has no effect on the brake pressure. The ECU commands the modulator valve to either:

• change the air pressure to the brake chamber, or

• hold the existing pressure.

However, it cannot automatically apply the brakes, or increase the brake application pressure above the level applied by the driver.

The modulator valve typically contains two solenoids. The modulator valve and relay valve may be incorporated into a single unit. The modulator valve may also be separate, inserted into the service line to the brake chamber(s) after any relay valve, located as close as practicable to the chamber(s) itself.

When the modulator valve is separate, it has to control more air flow and, therefore, includes two larger diaphragm valves which are controlled by the solenoids. It usually has three ports: the supply port, the delivery port and the exhaust port.

• The supply port receives air from a quick release or relay valve.

• The delivery port sends air to the brake chambers.

• The exhaust port vents air from the brake chamber(s).

Typically, when an ECU controlling a separate modulator valve detects impending wheel lockup, it activates the solenoids to close the supply port and open the exhaust port. When enough air is vented to prevent wheel lockup, the exhaust valve will close and the ECU will—depending on the situation—either:

• keep the supply port closed to maintain existing pressure, or

• open the supply port to allow brake application pressure to increase and repeat the cycle.

Wheel Speed Sensors

The wheel speed sensor has two main components: the exciter and the pickup. Other components include associated wiring and mounting equipment. Exciter—

The exciter is a ring with notched teeth. The most commonly used exciter has 100 evenly spaced teeth, but the number of teeth can vary depending on the system design. The component is known by several names: sensor ring, tooth wheel, tone ring, and exciter.

Pickup—The pickup is commonly called “the sensor.” It contains a wire coil/magnet assembly, which generates pulses of electricity as the teeth of the exciter pass in front of it. The ECU uses the pulses to determine wheel speeds and rates of acceleration/deceleration. The strength of these electrical pulses decreases rapidly with slight increases in the gap between the pickup and the exciter.

Wheel speed sensor location varies. It can be located anywhere on the axle to sense wheel speed. The sensor can be an assembly containing both the exciter and the pickup with a fixed gap. Or, the pickup and the exciter can be mounted separately on different parts of the axle assembly. The sensor pickup is a sealed unit and typically of elbow or straight design.

On most ABS air-braked vehicles, the pickup is located in the mounting flange on the wheel end. The exciter usually is either mounted on—or integrated with—the wheel hub.

Since the output of the pickup decreases so rapidly with slight increases in exciter-pickup gap, it is imperative that the wheel end and sensor gap be maintained within the manufacturer’s specification.

When the wheels of only one tandem axle have wheel speed sensors, they are usually placed on the axle whose wheels are most likely to lock-up first during braking. On a tandem with a four-spring suspension, the sensors are generally on the lead axle. On a tandem with air suspension, the sensors are generally located on the trailing axle.

ABS configuration is defined by the arrangement and number of sensors and modulator valves used. The most common configurations for power units are:

• four sensors/four modulators (4S/4M),

• six sensors/four modulators (6S/4M), and

• six sensors/six modulators (6S/6M).

ABS Malfunction Indicator Lamps

Vehicles required to have an ABS must have ABS malfunction indicator lamps. These lamps must be yellow and light up when the ABS has a “malfunction that affects the generation or transmission of response or control signals” in the ABS.

ABS malfunction indicator lamps are not required to light up for every type of malfunction. However, they are required to light up for short periods of time for a bulb check whenever the ABS starts to receive electrical power. The warning lamps for trailers and dollies are not required to light up for a bulb check unless the vehicle is stopped.

In-cab ABS indicator lamps are typically located on the instrument panel. The exact location and appearance vary by vehicle/component manufacturer. Consult the manufacturer’s service information for specifics.

ABS Diagnostics

Although not required by law, all air brake ABSs have self- diagnostic capability. On truck-tractors and single-unit or straight trucks, an ABS provides this information to technicians through the malfunction indicator lamp and/or an electronic diagnostic tool, which plugs into an on-board diagnostic connector. The connector is typically located inside the tractor cab just underneath the left end of the instrument panel. It is usually the same connector that’s used to troubleshoot electronic engines.

Error Detection Methods

One ABS benefit is the ability to electronically detect component or system failures. This electronic detection occurs either during self-test checks at start-up, or during continuous passive monitoring.

At start-up, the ECU will activate the ABS malfunction indicator lamp and briefly energize the ABS modulator valves (ignition-on blowdown or “chuff test”). At the same time, the ECU checks the wheel speed sensors and other essential components for proper operation. If no problems are found, and the ECU detects that wheel speed sensors were functioning properly just prior to the last vehicle shutdown, the ABS malfunction indicator lamp will go out. On earlier systems, the lamp would not go out until the vehicle reached about 5 mph.

During vehicle operation, the various ABS components also continually monitor each other for failures and “out-of-range” operating parameters. Through this process, the ECU detects abnormalities during operation and activates the ABS malfunction indicator lamp as appropriate.

The ECU will generally detect two types of faults: active and stored. An active fault is a current and continuous failure in need of repair (such as a broken connector). A stored fault is a failure that affects ABS operation intermittently (such as a loose connector).Technicians typically can retrieve failure information either through blink codes or an electronic diagnostic tool. For explanations of manufacturer-specific diagnostic and troubleshooting tools and procedures, consult the appropriate manufacturer’s service information.

Top 10 Most Commonly Encountered Problems That Trigger ABS Malfunction Indicator Lamps

1. Abraded or cut wires in convuluted tubing near frame clamps.

2. Cut or corroded wires near sharp frame members and frame-mounted modulators.

3. Wire jacket worn through from overlapping sensor and modulator wires near frame members and frame-mounted modulators

4. Corroded connectors and connections not properly sealed or damaged seals.

5. Damaged connector latches or connectors not completely sealed to mating assemblies.

6. Terminals not completely latched or seated into connectors

7. Excessive sensor air gap, sensor clip tension or excessive wheel bearing endplay.

8. Damage to exposed wires exiting or entering wire tubing.

9. Worn, chipped or damaged sensor or modulator.

10. Non-functioning controller (ECU).




Even if the space between the teeth of the exciter ring is full of dust and particles from the brake linings, the monitoring operation is not affected. In fact, the magnetic property of the dirt in the gaps is similar to that of air. The change in the magnetic field is determined by the spacing of the teeth of the exciter ring. The output voltage is unaffected by dirt.

Therefore, an ABS fault cannot be remedied by cleaning the tooth wheel with compressed air. Dirt in the gaps doesn’t affect voltage output, so removing it will not remedy an ABS fault.


CAUTION: Follow all recommended safety warnings and cautions. To prevent eye injury, always wear safe eye protection when performing maintenance or service. Do not work under a vehicle supported only by jacks. Jacks can slip or fall over and cause serious personal injury.

To adjust the ABS sensor pickup, gently push the sensor pickup in until it contacts the tooth wheel:

• On the steering axle, the sensor pickup may be acces- sible on the in-board side of the steering knuckle.

• On the drive axle, the wheel and drum assembly must be pulled to gain access to the pickup. Prior to pulling the wheel and drum assembly, observe the output voltage of the pickup while rotating the wheel by hand. The amount of output voltage is dependent upon the sensor pickup gap and wheel speed. Refer to the manufacturer’s recommendations for proper voltage levels.


2. ABS Sensor Pickup Removal & Installation

The following installation and removal procedure is a guideline only. When removing or installing a sensor pickup on your system, always follow the procedures detailed in the manufacturer’s maintenance manual.


3. Sensor Pickup Removal—Front Axle

To remove the sensor pickup from the front axle:

1. Put wheel chocks under the rear tires to keep the vehicle from moving. Apply the parking brake.

2. Remove the pickup and spring clip from the steering knuckle. Use a twisting motion if necessary. Never pull or tug on the cable.

3. Disconnect the pickup cable from the chassis harness. Be careful not to criss-cross wiring.


4. Sensor Pickup Installation—Front Axle

To replace the sensor pickup in the front axle:

1. Connect the sensor cable to the chassis harness. Be careful not to criss-cross wiring.

2. Install the fasteners used to hold the sensor pickup cable in place.

3. Apply lubricant to the sensor spring clip and to the body of the pickup. NOTE: Use a mineral oil-based lubricant that contains molybdenum disulfide. The lubricant should have excellent anti corrosion and adhesion characteristics, and be capable of continu- ously functioning in a temperature range of -40° to 300° F (-40° - 150° C).

4. Clean and inspect the hole in the steering knuckle. Install the sensor pickup spring clip. Make sure the flange stops are on the inboard side of the vehicle.

5. Push the sensor spring clip into the bushing in the steering knuckle until the clip stops.

6. Push the sensor pickup completely into the sensor spring clip until it contacts the tooth wheel/exciter.

7. Install fasteners and straps to retain the pickup wiring.

8. Remove the wheel chocks.


5. Sensor Pickup Removal—Rear Axle

1. Put chocks under the front tires to keep the vehicle from moving.

2. Raise the rear tire off the ground. Put safety stands under the axle.

3. Release the parking brake and back off the slack adjuster to release the brake shoes.

4. Remove the wheel and tire assembly from the axle.

5. Remove the brake drum.

6. Remove the pickup from the mounting block in the axle housing. Use a twisting motion if necessary. Never pull or tug on the cable.

7. Remove the sensor spring clip from the mounting block.

8. Disconnect the fasteners that hold the sensor cable and the hose clamp to the other components.

9. Disconnect the pickup cable from the chassis harness.


6. Sensor Pickup Installation—Rear Axle

To reinstall the sensor pickup in the rear axle:

1. Apply lubricant to the sensor spring clip and to the body of the pickup. Follow manufacturer’s recommended lube specification (See lubricant recommendation in previous section “ Sensor Pickup Installation—Front Axle”).

2. Clean and inspect the hole in the mounting block. Install the sensor spring clip. Make sure the flange stops on the inboard side of the vehicle.

3. Push the sensor spring clip into the mounting block until it stops.

4. Push the pickup completely into the sensor spring clip until it contacts the tooth wheel. See figure at left.

5. Insert the pickup cable through the hole in the spider and axle housing flange. Route the cable to the frame rail. Be sure to route the cable in a way that will prevent pinching or chafing, and will allow sufficient movement for suspension travel.

6. Connect the pickup cable to the chassis harness.

7. Install the fasteners that hold the pickup cable in place.

8. Install the brake drum on the wheel hub.


7. Proper ABS Sensor Resistance

For most common types of ABS sensors, the sensor circuit resistance is between 700-3000 ohms. Resistance can be measured at the sensor connection when it is removed from the ECU, or right at the sensor when the extension cable is removed. Follow the manufacturer’s specifications to determine the correct sensor resistance.


8. Modulator Valve/Routine Inspection

As part of a routine vehicle preventive maintenance program, ABS modulator valves should be checked for proper operation and condition. This inspection generally should include:

1. Removal of contaminates and a visual inspection for excessive corrosion and physical damage.

2. Inspection of all air lines and wiring harnesses for signs of wear or physical damage.

3. Testing for leakage and proper operation. For specific modulator valve inspection and testing procedures, consult the manufacturer’s service information.


9. Modulator Valve Removal and Installation

The following removal and installation information is offered as a guideline only. Always refer to the manufacturer’s specific instructions when removing or installing ABS modulator valves. Removal

1. Disconnect the harness connector from the modulator valve. Be careful not to criss-cross wiring.

2. Disconnect the air supply and air delivery lines from their respective ports.

3. Remove modulator valve mounting fasteners.

4. Remove the modulator valve. Installation

1. Install the modulator valve with appropriate mounting fasteners. Tighten to specified torque.

2. Connect the air supply and air delivery lines at their respective ports.

3. Connect the harness connector to the modulator valve. Be careful not to criss-cross wiring.

4. Check installation by applying the brakes, listening for leaks at the modulator valve.

5. Turn the ignition on, and listen for the modulator valve to cycle. If the valve fails to cycle, check the electrical connection and any stored or active fault codes. Drive the vehicle to verify that the ABS and its malfunction lamp operate properly.


10. Proper ABS Modulator Valve Resistance

For most ABS modulator valves, the resistance range between each valve solenoid coil terminal and the ground on the ABS valve connector is between 3-10 ohms. To test this resistance, disconnect the wiring connector from the modulator and test the resistance between the two pins of each solenoid. Follow the manufacturer’s instructions for determining valve resistance.

© Barry Brideau 2003-2017